Best Carb for Modded 306 cid - Page 3 - Vintage Mustang Forums
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post #31 of 53 (permalink) Old 06-26-2019, 07:50 PM
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Good info sharing going on. I didn't mention that my compression ratio is 10:1, used Keith Black Flat top pistons with the smaller chamber volume 165 heads. 600 CFM is looking to be a good size between the 570 and 650, I did notice one build using a 625 cfm when I did a search, I was just looking at my Holley 570 4 barrel which is basically new. I guess I could sell it, if I end up not using it.
On the 9:1 306 with Trick Flow heads I mentioned above, I started out with a 650 double-pumper, on the suggestion of my engine builder. As others have suggested, heís a drag race guy.....enough said.

I was never happy with the way it behaved in the real world, even in a VERY light car (2,000 lbs) and a manual trans.

Iíve since switched to a 570 Street Avenger, vacuum secondary, and while I canít say itís the PERFECT carb for my application, Iím much happier than I was with the double pump.

As someone else said, be honest with yourself about what youíre going to be doing with this car.
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post #32 of 53 (permalink) Old 06-26-2019, 11:13 PM
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On the 9:1 306 with Trick Flow heads I mentioned above, I started out with a 650 double-pumper, on the suggestion of my engine builder. As others have suggested, heís a drag race guy.....enough said.

I was never happy with the way it behaved in the real world, even in a VERY light car (2,000 lbs) and a manual trans.

Iíve since switched to a 570 Street Avenger, vacuum secondary, and while I canít say itís the PERFECT carb for my application, Iím much happier than I was with the double pump.

As someone else said, be honest with yourself about what youíre going to be doing with this car.
But, did you have that lean surge with the double pumper?

Of course, that's not the end all, be all - and there is a lot more to all this than how a lean spot was resolved. I am glad you solved the problem and are happy with the results!

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post #33 of 53 (permalink) Old 06-26-2019, 11:26 PM
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But, did you have that lean surge with the double pumper?

Of course, that's not the end all, be all - and there is a lot more to all this than how a lean spot was resolved. I am glad you solved the problem and are happy with the results!
So, letís hear it, what would you suggest for curing a lean surge?
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post #34 of 53 (permalink) Old 06-26-2019, 11:51 PM
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So, letís hear it, what would you suggest for curing a lean surge?

Without being there, knowing your needs and driving style, etc - I would have started by adding fuel. The previous double pumper's secondary accelerator pump was probably enough, and while mentioned you had other issues/things you didn't like it, I'm guessing a lean surge with that carb wasn't one of them.

Your way obviously worked too - ultimately you either take away air (stiffer secondary spring) or add fuel (accelerator pump on the secondaries). :-)

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post #35 of 53 (permalink) Old 06-27-2019, 12:04 AM
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Your way obviously worked too - ultimately you either take away air (stiffer secondary spring) or add fuel (accelerator pump on the secondaries). :-)
Add an accelerator pump on a vacuum secondary carb?
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post #36 of 53 (permalink) Old 06-27-2019, 12:19 AM
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Add an accelerator pump on a vacuum secondary carb?

That would be tricky ;-) If jetting up the secondaries, adding a power valve, or changing that plate didn't work, I'd go back to a double pumper. Maybe not the one you had, of course. There are options.

But if I had a warmed up small block with good flowing heads and everything else that took advantage of the improved flow (cam, intake, headers, etc), I'd be looking at a double pumper anyway. Call me a drag racer too but your engine guy wasn't wrong.

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post #37 of 53 (permalink) Old 06-27-2019, 02:31 AM
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Double pumpers are awesome on the strip. But vac secondaries work sort of like a 'variable venturi' carburetor. At low RPMs, when the engine is not pulling much vacuum at WOT, you get excellent atomization through the primaries. Then, as needed, you get more from the secondaries - again, with proper atomization.

If you're running a double-pumper with steep gears, and/or running a manual transmission, you get into higher RPMs so fast, there's not too much of a penalty for the poor mix quality; you build RPMs and then the carb can atomize well even with all 4 venturii roaring along.

On an automatic running highway gears with a stockish torque converter, you are often put in a situation where the initial spurt of gas from the accelerator pumps gets burnt, all 4 barrels are open, but the engine is still at low RPMs, and not drawing much air. At this point, most double pumpers will be raining big fat drops of gas down the intake, and even if you don't have a bog, that's not great for performance or for economy.

For an automatic-equipped car, destined for use on the street, I much prefer a carb that's going to deliver snappy performance no matter what my RPMs are at. I've seen some really nasty strip terrors that could not break the tires loose in first gear, from a rolling start (without dipping the clutch). I've used a lot of carburetors, chasing performance, economy, or both - but always with street use in mind. My favorite by far is the old Autolite 4100 and its newer descendants. I used to really make my buddies mad. Lining up against their 350 Rocket '72 Cutlass Supreme, 73 455 Buick Riviera, 71 400 GTO convertible, and '70 Super Bee 389, my little 302 would always stomp 'em, and they never could believe how eager my car was to 'go' from any speed. Though to be fair, Dave always really babied the clutch and granny-shifted his Bee.

A huge part of that was my old Autolite, because I sure didn't have the money for a torque converter or hot cam at the time.
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post #38 of 53 (permalink) Old 06-30-2019, 09:11 AM Thread Starter
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I appreciate the thoughts on keeping my existing Holley 570, I went to the site and used the carb calculator and the range is from 570-650. What about this carb? At least one hotrod build used it and it seemed to put out some come good numbers.

https://www.holley.com/products/fuel...rts/RDA-650-VS

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post #39 of 53 (permalink) Old 06-30-2019, 09:32 AM Thread Starter
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Originally Posted by Benny View Post
On the 9:1 306 with Trick Flow heads I mentioned above, I started out with a 650 double-pumper, on the suggestion of my engine builder. As others have suggested, heís a drag race guy.....enough said.

I was never happy with the way it behaved in the real world, even in a VERY light car (2,000 lbs) and a manual trans.

Iíve since switched to a 570 Street Avenger, vacuum secondary, and while I canít say itís the PERFECT carb for my application, Iím much happier than I was with the double pump.

As someone else said, be honest with yourself about what youíre going to be doing with this car.
I have been experimenting with different carb calculators and as you stated the 570cfm is what seems to be a good place to be since I'm not planning on spinning over 6000 rpms. The reason I went with the AFR 165's is because of how well they flow under .500. My cam Lunati 218/218 rpm range is 1800-5800, with the 1.7 rockers it gives me a little more lift so I'm thinking that the redline of the cam has shifted to at least 6000 rpms. With 10:1 compression ratio I'm thinking that I have plenty of power through the mid-range and top end. This is a C4 setup with a GV overdrive and 3.55:1 traction lock differential gears. 2400 stall converter

It will be used on the street and for spirited driving, but will never see a 1/4 mile strip or road course. Been there done that with my other vehicles and bikes. I just wanted a hopped up stang with 350-400hp.

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18' Tesla P3
17' Z06
13' M3

Previous fun toys
01' Twin Turbo Z06, 850 whp
06' CTS V LPE 427, 800 whp
99' Hayabusa
09' Vmax, Guru mod 240hp
65' and 95' Mustang GT
87' IROC and 2000 Camaro
91' Firebird Formula 350
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post #40 of 53 (permalink) Old 06-30-2019, 11:58 AM
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Originally Posted by Shinobi68 View Post
I appreciate the thoughts on keeping my existing Holley 570, I went to the site and used the carb calculator and the range is from 570-650. What about this carb? At least one hotrod build used it and it seemed to put out some come good numbers.

https://www.holley.com/products/fuel...rts/RDA-650-VS
Unless you are on a tight budget Get the quick fuel. Way nicer carb for a little more money. VS and electric choke Like this one. https://www.holley.com/products/fuel...arts/SS-680-VS

Last edited by cmefly; 06-30-2019 at 12:06 PM.
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post #41 of 53 (permalink) Old 06-30-2019, 01:02 PM Thread Starter
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A lot or recommendations for the QF on this thread, what are the benefits of the QF over the Street Avenger?
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post #42 of 53 (permalink) Old 06-30-2019, 01:22 PM
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A lot or recommendations for the QF on this thread, what are the benefits of the QF over the Street Avenger?
The Quick Fuel is more adjustable/ adaptable/ tune-able with replaceable air bleeds and power valve restrictions. Overkill for most (98.5%) applications but nice to have if you need them, and you know what youíre doing.
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post #43 of 53 (permalink) Old 06-30-2019, 03:14 PM
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As I stated earlier, Quick Fuel Brawler 650 vacuum. Great value for the money. I love mine. Also comes in race and DP. I use the race version, (no choke).

'67 coupe, 390, 4spd.
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post #44 of 53 (permalink) Old 06-30-2019, 05:53 PM
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Originally Posted by GypsyR View Post
...It's personal preference, but I like a "sporty" feel from my cars. Quick throttle response at any RPM is a big part of that...

Old school rule of thumb is not to use a double pumper unless the car has a manual transmission or a very high stall converter.
Give me some feedback on that double pumper rule of thumb, Gypsy or anyone with know-how, because I like quick throttle response as well. My 302 came with a 1406 Edelbrock 600 with vac secondaries and e-choke, AND a C4 auto.

I'm converting it to a roller 302, T5z manual, 3.73 rear end and my 302 has 170 TFS TW heads, B303 cam and 1.7 roller rockers, and I will likely only once or twice drag it just to see, but it's going to be a tight handling road course/street car, hopefully, when I'm done. Dual plane Edelbrock manifold.

No emotional connection to the Edelbrock, almost pulled the trigger on a FiTech, Sniper, multi-port EFI but held off, figured too many changes at once, let's break it in with a carb?

Should I stick with the 1406 or is the Summit 600 with 'annular' (whatever that is?) boosters THAT much better, that I might like it so much I don't bother with EFI for a while?

Crapola! Looks like the Summit carbs are vac secondary only?!?

Last edited by Fishfreq; 06-30-2019 at 05:56 PM.
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post #45 of 53 (permalink) Old 06-30-2019, 06:14 PM
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“Crapola! Looks like the Summit carbs are vac secondary only?!?”

I’d say start with the Edelbrock first.
And if you feel it could use a little more, get a Holley 650 double pumper.
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